The trend in recent years in the electronics industry has been to replace conventional leaded systems with leadless components, e.g. chip capacitors, resistors, diodes and transistors. Such leadless components are attached to printed wiring boards, e.g. fiberglass/epoxy or paper/phenolic boards, in an automatic packaging system. Conventionally, the chips are adhered to the board and treated with solder flux. The board is then inverted and the electrodes of the chips wave soldered to the board at, e.g., 240.degree. C. for six seconds.
The adhesive utilized to adhere the leadless components, i.e. chips, to circuit boards is particularly critical since the loss of a single chip per thousand cannot be tolerated. The adhesive must be highly reliable so that no chip will drop from the circuit board when it is inverted and wave soldered. In addition, the adhesive must conform readily so that the chip lays flat on the board, be solventless to minimize pollution, be easily dispensed for high speed operations, be long-lived and fast-acting.
Conventionally, chips are attached to circuit boards by heat curing or baking a small amount of adhesive, usually an epoxy resin, placed either on the board or the component before the latter is placed on the board. In present commercial operations, a substantial time, i.e. up to 20 minutes, in an oven may be required to effectively adhere leadless components to a printed circuit board. This baking step is disadvantageous for a number of reasons.
The baking step is an interruption of the manufacturing process and, in effect, negates high speed automated procedures which have been developed, for example, for accurately placing the chips on the boards. An extended heat cure may also cause the adhesive to flow and thin out under the chip. This reduces the effectiveness of the adhesive and may allow the chip to move from its position.
A recent improvement in adhering leadless components to circuit boards is a commercially available apparatus which permits an on-line, two-stage, i.e. ultraviolet light and heat, cure of an adhesive which is basically an acrylated epoxy resin. This apparatus effectively eliminates most of the problems inherent in the conventional heat cure because the partial cure affected by ultraviolet light acts to hold the chip in place, prevent adhesive flow and exclude oxygen which can adversely affect the subsequent thermal cure. A cure with ultraviolet light, however, is only effective where the light can reach and penetrate the resin. Therefore, fillers which would add strength to the adhesive and reduce costs cannot be utilized in conventionally effective amounts since, as most are opaque substances, such amounts would retard the penetration of ultraviolet light.
European Patent application No. 16,984 discloses an adhesive composition for mounting electronic components which is curable by ultraviolet light alone or in combination with subsequent heat curing. It is disclosed that it is essential that a portion of the adhesive composition extend beyond the chip so that ultraviolet light can strike it. It is stated that curing then takes place throughout the adhesive by a "chain-reaction".